U.S. Pat. No. 4,793,703 (Fretz, Jr.) describes a three-layer, glass-plastic, laminated ophthalmic lens structure. That lens is composed of (1) an inorganic glass element, preferably photochromic glass, (2) a rigid, organic plastic element and (3) an interlayer of flexible, organic adhesive, either thermosetting or thermoplastic. The plastic element is selected to provide a coefficient of thermal expansion in the range of 200.degree.-700.times.10.sup.-7 /.degree. C. This alleviates severe stress conditions that tend to develop with higher expansion plastics, such as CR-39, marketed by PPG Industries, Pittsburgh, Pa.
An improvement in the three-layer, laminated lens structure of the -703 patent is described in U.S. Pat. No. 5,014,712 (Fetz, Jr.). In accordance with that improvement, the adhesive interlayer is more tightly bonded to the glass and plastic elements by providing reactive groups on the surfaces of the elements and the adhesive interlayer. This controls delayed delamination.
There are various advantages obtained by using an epoxy resin system for the plastic element in the laminated glass-plastic lens. These include low coefficient of thermal expansion, and a high Tg which favors stability during thermal processing. Pending U.S. application Ser. No. 07/589,403, filed Sep. 28, 1990, now U.S. Pat. No. 5,116,684, in the names of E. R. Fretz, Jr. et al. and assigned to the same assignee as this application describes an improved cast, epoxy plastic element and casting method. The improvement avoids striations in the casting. It derives from use of a casting mix in which the ingredients are maintained in certain relationships to each other.
Heretofore, it was thought necessary to produce the three elements of a laminated lens structure separately, and then assemble and process them into a unitary body. Technically, this procedure has been found eminently satisfactory. However, from a practical standpoint, it has proven time consuming and unduly expensive.
This led to a study of possible alternative methods that might prove more cost effective. In particular, the search was aimed at a method of forming a laminated lens blank in a continuous operation with a direct casting step. It is then a basic purpose of the present invention to provide such a simplified, less expensive process.